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analyze c6 clay body

updated sat 13 oct 01


Jim Larkin on fri 12 oct 01

After rereading my post to you concerning your teachers clay body, I =
realized I could have given a better answer. I expect she did tweak this =
clay body to fit the glazes she is using. Being in Missouri, Hawthorne =
fireclay would be a good, cheap choice for the core of the clay. Ball =
clay would be added for plasticity. The grog would be added to make it =
more all purpose (but I sure wouldn't put it on top of a stove), the =
redart would give it color (but that much at cone 10 reduction can =
sometimes lead to blackcore problems--some potters will use a percentage =
of iron oxide instead). All those things can be tweaked. But the most =
tweaking will come in the silica and feldspar additions. If you haven't =
seen it, you might want to look up the excellent article by Jim Robinson =
in Studio Potter, Volume 16, #2 (1988) called "Body Building For =
Potters: A Clay-Blending Formulary". Basically he uses 10 test glazes, =
ranging from low to high expansion, to determine the right balance of =
silica and feldspar to use in a clay body. Depending on the core clays =
used, too much silica, either in the clay or added, will cause shivering =
of glazes, even pot shattering. Too much feldspar and crazing will =
occur. Tweaking this balance for the clays you are using, along with =
having a general idea of the coefficient of expansion of the glazes you =
are using, will solve a host of clay/glaze problems.

Even though I and many other potters have used variations on this clay =
body for many years, I believe it can be improved by expanding the clay =
additions to the body. With just Hawthorne as the core clay, too many =
eggs are in one basket. Maybe the addition of a plastic kaolin, like =
Saffire, or some #6 tile clay,etc., would give a greater range of clay =
particle sizes, and also lessen the impact if something changes from the =
mine for any particular clay, which, over time, is a sure thing.

You are doing good to test this further for cone 6 use. You might find =
some nepheline syenite replacing some of the feldspar tightens it up if =
it has too much absorption. Happy testing!=20

Hope this helps.
Jim Larkin
Fox Pass Pottery
379 Fox Pass
Hot Springs, Arkansas